I have OS X El Capitan Version 10.11.6 which runs Parallels Version 12.2.1 (41615). I use it to work in Windows 10. Sometimes I can start Windows without problems. Often I need to do several attempts as Windows starts in a non-responsive mode ( no reaction to mouse clicks). However lately the situation got much worse. When Windows does start it appears is a small window, which seems to work. But whenever I try to maximize it to use both external monitors or even resize the window a tiny bit it immediately freezes. No amount of rebooting, tweaking the options or reinstalling parallels helps. I am considering getting rid of my Mac and getting a Windows machine as I still have to complete my Windows projects. Before I do that maybe someone can offers some hints of what could be wrong ?
